1. Technical Field
The present invention relates to an image sensor, more particularly to a lens shading compensation apparatus and a lens shading compensation method in the image sensor that compensate the difference in signal amplitude according to the position of pixels to preserve the quality of a primitive image.
2. Description of the Related Art
Recently, portable devices having an image sensor (e.g. digital cameras, mobile communication terminals, etc.) have been introduced in the market. The image sensor is composed of an array of photosensitive diodes called pixels or photosites.
The pixel itself usually does not extract colors from light, but converts photons in a broad spectrum band into electrons. In order to record color images by a single sensor, the sensor is filtered such that different pixels receive different colors. This type of sensor is known as a color filter array (CFA). Different color filters are arranged across the sensor according to a predefined pattern.
As a most common pattern, a Bayer pattern is widely employed in the CFA. In the Bayer pattern, a half of the total number of pixels is green (G), and two quarters of the total number are assigned to red (R) and blue (B). In order to obtain color information, red, green and blue filters are arranged in a particular sequence to form a repetitive pattern. The Bayer pattern is composed of a 2×2 array.
The Bayer pattern is based on the premise that the human eye derives most of the luminance data from the green light. Therefore, an image with a higher resolution can be generated when more of the pixels are made to be green, compared to when an equal number of red, green, and blue pixels alternate.
However, the conventional image sensor in recently marketed portable devices had a problem of image distortion due to the geometric pattern of the pixel array. This is because of its small outer lens and high f number.
FIG. 1 illustrates the disparity in transmissivity between the central part and the periphery of a lens. As illustrated in FIG. 1, the pixels in the central part and the pixels in the periphery of the CFA of the image sensor are exposed to a light source from different positions.
These minute differences in position cause differences in illumination, and the differences in illumination affect color because of differences in light frequency and refractive index. Consequently, color distortion and reduction in signal amplitude dependant on the position of the pixels inevitably occur, degrading the quality of primitive images. Such a phenomenon is referred to as lens shading phenomenon.
FIG. 2(a) and FIG. 2(b) are diagrams illustrating the center points and characteristics of lens shading images formed according to red (R), green (G) and blue (B) lights. As shown in FIG. 2(a), a center point 210 of a pixel array 200 does not coincide with any of center points 213, 216, 219 of the lens shading images 203, 206, 209 formed on the pixel array 200 of an image sensor according to red, green and blue lights that passed through the lens. Furthermore, as shown in FIG. 2(b) it is noted that the lens shading images have different characteristics 223, 226, 229 of the lens shading images.
In order to compensate the lens shading phenomenon has been introduced a method that equalizes the brightness of all the pixels of the pixel array after photographing an image of a white area.
FIG. 3 is a flow chart showing a conventional image processing method for making the brightness uniform.
Referring to FIG. 3, after a primitive image is inputted (S310), black level compensation (S320) and noise removal (S330) are performed. Subsequently, lens shading compensation (S340) and color interpolation (S350) are performed. Then, the quality of images is improved (S360) through color adjustment, gamma conversion, format conversion, etc., and image data is finally outputted to a display unit (S370).
However, he above lens shading compensation (S340) is applied under a premise that the lens shading images of red, green and blue colors have the same shape and center points. Moreover, prior to the color interpolation S350, each of the pixels of the pixel array has a pixel value on any one among red, green and blue colors in accordance with the Bayer pattern, and neighboring pixels have each different color component, so that it is difficult to compensate the lens shading image for each color without an interruption of other colors.
FIG. 4(a), FIG. 4(b), and FIG. 4(c) illustrate lens shading images of another embodiment.
Referring to FIG. 4(a), a center point 415 of a lens shading image 405 is deviated to the left with respect to a center point 410 of a pixel array 400. Referring to FIG. 4(b), the lens shading image is either tilted 425 or not tilted 420 against a longitudinal direction of the pixel array 400. Referring to FIG. 4(c), the lens shading image 405 is either tilted 435 or not tilted 430 against a lateral direction of the pixel array 400.
When tilted along the longitudinal direction 425 or the lateral direction 435, the lens shading image 405 cannot have a uniform luminance. However, there has not been proposed yet a method to compensate the tilting characteristic of the lens shading image 425 or 435.